Accompanying FIG. 1 is a diagram showing the transmission of telephone calls or data via a satellite operating in TDMA mode.
As shown in the figure, a first transmitter station 1 transmits a first packet of data A to a telecommunications satellite 2, which packet is received by the satellite between instants t and (t+t.sub.0), while a second transmitter station 3 transmits a second data packet B which is received by the satellite between instants (t+t.sub.0 +.epsilon.) and (t+t.sub.0 +.epsilon.+t.sub.1), where .epsilon. is a very short time interval which may tend, in the limit, to zero. The data packets A and B transit through the satellite 2 which re-transmits them one after the other towards a receiver station 4 as shown diagrammatically in the figure. The receiver 4 thus receives the packet A followed by the packet B after an intervening guard time interval equal to .epsilon..
The data packets A and B which thus pass through the satellite 2 come from different directions and therefore do not follow identical paths. They may therefore be subject to different fading. For example, if weather conditions are unfavorable over transmitter station 3, e.g. by virtue of a zone of rain 5, the packets A and B willl not be of the same amplitude on arrival at the satellite 2. And as a result the receiver station 4 has to demodulate signals which include a wide range of levels.
By way of specific example, the INTELSAT and EUTELSAT satellite telecommunications networks are designed to tolerate successive packets having levels which differ by as much as 5 dB, with the level of each packet lying somewhere in the range +2 dB to -10 dB relative to the nominal reception level.
It can thus happen that the receiver station 4 receives signals A or B which are either at too high a level or at too low a level, thereby increasing the error rate on reception of each signal, and thus degrading transmission. As is explained below, it is not a priori possible to provide the receiver 4 with a conventional automatic gain control (AGC) circuit as is used for conventional traffic receivers, with the result that the sole solution existing heretofore consists in correcting the level differences in an experimental manner. It is thus conventional for an operator located at receiver station 4 to make use of service channels to warn transmitter stations 1 and/or 3 of the poor reception quality of signals transmitted therefrom, and thus ask them to temporarily increase or decrease the level at which the signals are transmitted.
Naturally such an experimental procedure is unsatisfactory for quality of reception since the response time is necessarily rather long, and also since it is subject to human failings, and it would therefore be desirable to provide an AGC circuit for the receiver 4.
Unfortunately, as mentioned above, a conventional AGC cannot be used in such a receiver. Conventional AGC circuits have a relatively long response time and act on signals whose power corresponds to an average of the received signal powers. Under such conditions, any signal packets A or B which correspond to a lower power than average, either because of their low amplitude or because of their short duration, or for both reasons, would have little or no effect on the AGC which would operate as a function of the more powerful packets A or B, such that packet demodulation would continue, overall, to occur under poor conditions.
Naturally the use of a fast AGC is not, a priori, possible either since such fast correction, even if it could be provided, would tend to level out the amplitude of the signals themselves, thereby degrading transmission. Further, so far as the Applicant is aware, such high-speed AGCs are not available at present, and in particular for the reason mentioned.
Also, in TDMA each transmitted and received data packet (A or B in FIG. 1) comprises in succession: an acquisition sequence or preamble, followed by data which constitutes the useful signal and is generally in digital form. For example, in the INTELSAT and EUTELSAT systems, a data packet A or B comprises, in succession and as outlined in accompanying FIG. 2, a three-portion preamble 6 and a data portion 10. The preamble 6 comprises:
a first portion 7 constituted by an unmodulated wave whose frequency is equal to the carrier wave frequency, e.g. 140 MHz in the intermediate frequency band, said first portion being of fairly short duration, e.g. about 1 microsecond, and being intended to allow the carrier frequency to be recovered in the receiver's demodulator;
a second portion 8 of carrier wave including phase shifts to enable the clock frequency to be recovered by the demodulator, with the carrier wave being practically fully modulated by a sinusoidal wave of twice the clock period, in a manner which is explained in greater detail below; and
a last portion 9, constituted by a recognition word or "unique word" which is digitally encoded and which serves, for example, to remove ambiguity on the phase state of the carrier which corresponds to level 00 (when using four-state phase modulation, for example), or in other words serving to provide an absolute phase reference on reception.
The data portion 10 is generally longer than the entire preamble 6 and may have a duration of 3 us for example, said portion comprising the entire data content of the packet and being constituted by a succession transmission symbols.
Preferred implementations of the present invention provide a method and a circuit enabling automatic gain control to be provided for a time division multiple access receiver.